In some cases, electronic devices of various types that generate heat while operating are provided with a cooling unit to avoid an influence of heat generated therein on a neighboring component. For example, in an LSI (Large Scale Integrated) circuit and/or IC (Integrated Circuit) to be used in an electronic device such as a computer or the like, circuit integration is increasing in an accelerated manner in each circuit generation and, as a result, an amount of heat generated therein tends to increase. To operate the LSI circuit and/or IC stably at high speed, it is necessary to control an operating temperature so as to be a predetermined temperature or less and, therefore, a cooling method corresponding to the amount of heat generated by the LSI circuit or IC is to be employed. However, in recent years, high speed signal transfer between the LSI circuit and/or IC and their peripheral parts rather than a high speed operation of the LSI circuit and/or IC themselves is more contributable to an improvement in operation speed of a computer. High speed signal transfer seems to be advantageously realized by shortening wiring length, however, the shortening of the wiring length makes it difficult to ensure space for mounting a cooling unit having a sufficient size corresponding to the amount of heat generated therein.
As a cooling method for an LSI circuit, a method is known in which a small cooling unit is mounted on the LSI circuit and a coolant housed in the cooling unit is circulated by a pump into a heat radiating portion with wider space so that heat by the cooling unit is transferred to the heat radiating portion and is then heat radiated into environmental air. Moreover, a heat pipe has been actually used that is configured to circulate a coolant after bringing about a phase change in coolant from liquid to vapor and then heat is transferred to a condensation chamber by utilizing a pressure difference caused by volume expansion. In the cooling method bringing about a phase change in coolant, higher cooling efficiency can be obtained compared with a method in which cooled water is circulated by a pump.
Specifically, this heat pipe does not use a tool requiring power such as a pump and replacement due to maintenance and/or life is not necessary and, as a result, is widely used. In the heat pipe, after evaporation of a coolant in a liquid phase in an evaporation chamber, the evaporated coolant is again cooled in a condensation chamber, resulting in radiation of heat and then a phase change in the coolant from vapor to liquid occurs and the resulting liquid phase coolant returns back into the evaporation chamber. The method in which the liquid phase coolant, in the circulation of the coolant, is returned back to the evaporation chamber by using gravity is called a heat-siphon type heat pipe or a boiling and cooling device.
Examples of the above cooling unit are shown in FIG. 1 of Related Art Patent Reference 1, FIG. 4 of Related Art Patent Reference 2, and FIG. 1 of Related Art Patent Reference 3. In the cooling structure disclosed in the Related Art Patent References 1 to 3, as shown in the schematic diagram in FIG. 12, coolant in a liquid phase state (hereinafter, may be simply referred to as liquid phase coolant) L housed in an evaporation chamber 101 in a cooling unit 100 comes into contact with a boiling surface 101a to evaporate therein, resulting in production of coolant in a vapor phase state (hereinafter, may be simply referred to as vapor phase coolant) V, which cools the boiling surface 101a. The vapor phase coolant V produced in the evaporation chamber 101, as shown in FIG. 12, rises by buoyancy and passes through a vapor pipe (vapor passage) 102 mounted above the evaporation chamber 101 and enters a condensation chamber 105 to be cooled and is restored to the liquid phase coolant L. The liquid phase coolant L restored to a liquid phase after heat radiation in the condensation chamber 105, as shown by the arrows in FIG. 12, passes through a liquid return pipe (liquid return passage) 103 mounted or the evaporation chamber 101 and returns back to the evaporation chamber 101 for circulation.
However, cooling efficiency of the cooling structures described in the Related Art Patent References 1 to 3 is not enough to cool an LSI circuit and/or IC which generates a large amount of heat. The reason for the above is that both the vapor port 102a of the vapor pipe 102 and liquid return port 103a of the liquid return pipe 103 exist in a same direction of the evaporation chamber at the time of circulation of coolant in the cooling structure and, as a result, the vapor phase coolant V rising upward as vapor strikes the liquid phase coolant L dropping downward as liquid. That is, the vapor phase coolant V rising by buoyancy and the liquid phase cool ant L dropping downward by gravity move in a direction opposite to each other and interfere with the movement of the coolant, thus making it difficult for the vapor phase coolant V to escape into the condensation chamber 105, resulting in an increase of pressure in the evaporation chamber 101. The increase in internal pressure in the evaporation chamber 101 causes an increase in saturated vapor pressure of the coolant, which raises its boiling point. Therefore, cooling efficiency is lowered.
Moreover, in FIG. 4 of Related Art Patent Reference 4, a cooling unit is configured with an integrally constructed evaporation chamber and condensation chamber without using a vapor pipe and a liquid return pipe. According to this cooling unit, the coolant is partitioned into a liquid phase coolant and a vapor phase coolant in the evaporation chamber, whereby striking between the liquid phase coolant and vapor phase coolant is avoided and, as a result, a decrease in cooling capability can be prevented.
In FIGS. 2, 5, and 8 of Related Art Patent Reference 5, a cooling unit is disclosed in which an evaporation chamber and a condensation chamber are integrally constructed and a partitioning unit is mounted between the evaporation chamber and condensation chamber, thereby avoiding the decrease in coolant capability and enabling the circulation of the coolant. In this cooling structure, the vapor phase coolant is transferred through a vapor pipe from the evaporation chamber to the condensation chamber to bring about a phase change in a liquid phase coolant, which is then returned back to the evaporation chamber from the condensation chamber formed above the evaporation chamber.
In FIG. 2 of Related Art Patent Reference 6 and in FIGS. 1 and 2 of Related Art Patent Reference 7, a cooling unit is disclosed in which a liquid return port of a liquid return pipe extending from a condensation chamber is allowed to enter the liquid phase coolant housed in an evaporation chamber, whereby the decrease in cooling capability is avoided and the coolant is circulated. In this cooling structure, the liquid phase coolant cooled in the condensation chamber is returned back into the evaporation chamber without coming into contact with the vapor phase coolant that evaporates in the evaporation chamber. FIG. 3 of the Related Art Patent Reference 4 shows that cooling fins are mounted in a standing state to improve evaporating efficiency (cooling efficiency) in the evaporation chamber.
Furthermore, the Related Art Patent Reference 4 discloses that, when the liquid phase coolant coming into contact with a boiling surface of the evaporation chamber evaporates, a nucleus (cavity) of the vapor phase coolant in a small state is formed on a surface of the boiling surface. One example of the formation of a nucleus of the vapor phase coolant is shown in FIGS. 4 and 5 of Related Art Patent Reference 8. Generally, the generation of bubbles at time of boiling the nucleus is caused by non-uniform nucleus formation and the formation of a flaw and/or hollow on a boiling surface operating as a heat conduction surface become a cause for the generation of bubbles and, as a result, much more bubbles are generated which facilitates a phase change from liquid to vapor.